Integrated Development Approach For A Mature Oil Field

Abstract

Bahrain field, discovered in 1932, is a highly faulted asymmetrical anticline trending in North - South direction (Fig.1). There are as many as seventeen oil producing and three gas producing reservoirs within Jurassic to upper Cretaceous formations apart from the Khuff gas zones below in Permian limestone formations (Fig-2). These reservoirs occur with divergent lithology, rock and fluid properties. Some of them are saturated while some are highly under saturated. Some of them are oil wet and some water wet. They operate under various drive mechanisms and pressure maintenance schemes. Gas injection in Mauddud, which is one of the major oil producing zones in Bahrain field, has been in operation for the last 65 years. Gas injection in Arab-D is in progress from 1986. Pilot peripheral water injection is in progress in wara sands in NE area of the field since 1995. Additionally, different production mechanisms like gas lift, pumping and stop cocking for GOR control is applied. The field being mature and structurally complicated naturally demands an integrated approach for effective reservoir management to ensure maximum recovery. Due to application of diverse reservoir management techniques, reservoir characterization, Geo statistical application for anisotropy and increased accuracy for simulation, the field has been producing with minimum annual decline (Fig-3). By integrating the Geological, Seismic and reservoir / production data the productive limits of various reservoirs have been extended adding substantial reserves. This paper briefly discusses the approach adopted in the effective reservoir management of this giant mature oil field. Introduction Bahrain Field is one of the largest oil and gas fields in the world. The field has been on production for the last 70 years and has produced around 20% of the initial oil in place. Currently about 400 wells are producing from fifteen reservoirs. The effective reservoir management techniques which include Development Drilling, active work over operations and applying new production enhancement projects have provided the road map to success in arresting the decline rate. Data acquisition and analysis was the key to all the methods applied in Bahrain field (Fig-4). A program called Petro vision was utilized in computerizing the data from all the 700 wells drilled so far. Seismic data, reservoir rock typing and formation evaluation has given valuable insight into reservoir performance and future well work opportunities. The reviews also highlighted the need for development of certain marginal shallow reservoirs and location of infill wells in certain areas in view of the inadequate drainage due to geological complexities. This made it necessary to drill more wells and initiate some IOR techniques for some of the zones. The field is now under active development program and several wells are being worked over for zonal changes and GOR control. Dual Strings (shallow zones in short string &Bahrain zones in long string) completions have also aided in developing the shallow zones, which were not, developed thus far due to poor economics. With reserves gradually adding up from new areas, it is required to continue the current development not only within the known productive limits but also in the extended areas. At this stage, the suitability of Horizontal Drilling was evaluated for two reservoirs and found successful. This is leading to efficient exploitation of reserves and improved economics for development. Further, it is also planned to initiate Improved Recovery pilot projects soon to evaluate and assess the suitability of these techniques for Bahrain Field. By these methods, it is anticipated that some more oil now classified as "Hypothetical", may also be recovered in addition to the currently estimated remaining reserves.

Integrated Reservoir Connectivity Study of Ahmadi Fractured Reservoir in Bahrain Field

“…Furthermore, the fault throws and the areas of communication between Ab and the underlying zones controlled the degree of communication and the type of fluids being introduced into the Ab zones. Earlier studies has shown that Aab zone is continuously receiving fluid and pressure support in certain areas from the underlying higher pressure Mauddud zone (which is undergoing gas injection 8,9 ) through an intermediate sandstone layer, the Wara sandstone which exist in the form of erratic sand lenses distributed irregularly all over the field 6,7 .…”

“…The source of water in this reservoir is lower reservoirs. And, the connectivity with lower reservoirs is provided by both fracture corridors and a high-porosity-permeability "Ac sand reservoir beneath Ahmadi 8,9 . Oil and gas production data indicated that wells in north and south have performed much better than the crestal wells.…”

Mature Field Development - A Review

“…They include data acquisition/analysis campaigns for reservoir simulation to re-evaluate the remaining reserves 119,120 or revisiting/recompletion the wells 121 , re-engineering using classic reservoir engineering analysis techniques and clustering the wells based on their performances 122,123 . Another cost effective reservoir development practice was to improve the volumetric sweep efficiency through the realignment of the injection and production wells 124 .…”